The aims of this proposal are to (1) identify cellular changes in the developing avian brain that underlie "critical" or "sensitive" periods for vocal learning and, (2) to determine how such changes are influenced by the learning of song-like vocalizations. Most birds learn song memorizing a suitable song model (sensory learning) and then using auditory feedback to mimic that model (sensorimotor learning). These two phases of vocal learning are often restricted to species-specific developmental period believed to coincide with pivotal changes in the organization of song-related brain regions. In zebra finches, for instance, sensory and sensorimotor learning overlap with drastic changes in the number, size, and connectivity of song-related neurons. The first study proposed will employ a comparative neuroanatomical approach to define better the relationship between neural change and sensory or sensorimotor learning. Developmental changes in the anatomy of vocal nuclei will be measured in swamp sparrows, a species in which the two phases of vocal learning are well separated in time. Next, neural changes that may be necessary for memorizing a vocal model will be identified. Acoustic isolation will be used to extend the critical period for sensory learning in zebra finches. It will then be determined how this manipulation influences the timing of neural changes in developing song regions. Finally, to determine how song learning influences the organization of song nuclei, neuroanatomical measurements will be correlated with individual differences in song complexity. Also, the ability of auditory experiences to influence the growth, retention, or addition of neurons during sensory or sensorimotor learning will be assessed. Critical learning periods exist for phenomenon as diverse as language acquisition, social attachment and imprinting. The proposal's long-term goal is to understand the neural mechanisms underlying these periods of unique susceptibility and to determine how information may be stored through experience-dependent modifications of the developing nervous system.